and junction sites of the interlaced MgAl–LDH nanocrystals with a mean diameter of 7.0 nm (Fig. S5, ESI), implying their promising catalytic activity. Meanwhile, the reduced packing density (large void) and the less sharp edge of LDH platelet-like nanocrystals can be observed (Fig. 1c and e). To get more insight on structural information of Fe3O4@MgAl–LDH@Au, the HRTEM image was obtained (Fig. 1f). It can be observed that both the Au and MgAl–LDH nanophases exhibit clear crystallinity as evidenced by well-defined lattice fringes. The interplanar distances of 0.235 and 0.225 nm for two separate nanophases can be indexed to the (111) plane of cubic Au (JCPDS 89-3697) and the (015) facet of the hexagonal MgAl–LDH phase (inset in Fig. 1f and Fig. S6 (ESI)). The EDX data (Fig. 1g) indicate that the magnetic core–shell particle contains Au, Mg, Al, Fe and O elements. The Au content is determined as 0.5 wt% upon ICP-AES analysis.
Table 1 Recycling results on the oxidation of 1-phenylethanol
The VSM analysis (Fig. S7, ESI) shows the typical superparamagnetism of the samples. The lower saturation magnetization (Ms) of Fe3O4@MgAl–LDH (20.9 emu g_1) than the Fe3O4 (83.8 emu g_1) is mainly due to the contribution of non-magnetic MgAl–LDH coatings (68 wt%) to the total sample. Interestingly, Ms of Fe3O4@MgAl–LDH@Au is greatly enhanced to 49.2 emu g_1, in line with its reduced MgAl–LDH content (64 wt%). This phenomenon can be ascribed to the removal of weakly linked MgAl–LDH particles among the interlaced MgAl–LDH nanocrystals during the Au loading process, which results in a less densely packed MgAl–LDH shell as indicated by SEM. The strong magnetic sensitivity of Fe3O4@MgAl–LDH@Au provides an easy and effective way to separate nanocatalysts from a reaction system.
The catalytic oxidation of 1-phenylethanol as a probe reaction over the present novel magnetic Fe3O4@MgAl–LDH@Au (7.0 nm Au) nanocatalyst demonstrates high catalytic activity. The yield of acetophenone is 99%, with a turnover frequency (TOF) of 66 h_1,
which is similar to that of the previously reported Au/MgAl–LDH (TOF, 74 h_1) with a Au average size of 2.7 nm at 40 1C, implying that the catalytic activity of Fe3O4@MgAl–LDH@Au can be further enhanced as the size of Au nanoparticles is decreased. Meanwhile, the high activity and selectivity of the Fe3O4@MgAl–LDH@Au can be related to the honeycomb like morphology of the support Fe3O4@MgAl–LDH being favourable to the high dispersion of Au nanoparticles and possible concerted catalysis of the basic support. Five reaction cycles have been tested for the Au nanocatalysts after easy magnetic separation by using a magnet (4500 G), and no deactivation of the catalyst has been observed (Table 1). Moreover, no Au, Mg and Al leached into the supernatant as confirmed by ICP (detection limit: 0.01 ppm) and almost the same morphology remained as evidenced by SEM of the reclaimed catalyst (Fig. S8, ESI).
In conclusion, a novel hierarchical core–shell magnetic gold nanocatalyst Fe3O4@MgAl–LDH@Au is first fabricated via a facile synthesis method. The direct coating of LDH plateletlike nanocrystals vertically oriented to the Fe3O4 surface leads to a honeycomb like core–shell Fe3O4@MgAl–LDH nanosphere. By a deposition–precipitation method, a gold-supported magnetic nanocatalyst Fe3O4@MgAl–LDH@Au has been obtained, which not only presents high 1-phenylethanol oxidation activity, but can be conveniently separated by an external magnetic field as well. Moreover, a series of magnetic Fe3O4@LDH nanospheres involving NiAl–LDH and CuNiAl–LDH can be fabricated based on the LDH layer composition tunability and multi-functionality of the LDH materials, making it possible to take good advantage of these hierarchical core–shell materials in many important applications in catalysis, adsorption and sensors.
This work is supported by the 973 Program (2011CBA00508).
译 文
简易合成易回收的分层核壳Fe3O4@MgAl–LDH@Au磁性纳米粒子催化剂催化氧化醇类物质
一种新的核壳结构的Fe3O4@MgAl–LDH@Au纳米催化剂的制备只是通过Au离子负载在已合成的纳米粒子Fe3O4@MgAl–LDH球体的MgAl–LDH的表面上。这种催化剂表现出较好的氧化1-苯基乙醇的活性，而且其可以有效地利用外部磁场作用力进行回收。
在化学合成中，选择性氧化醇类物质是羰基化合物的一大重要转变。最近研究表明，水滑石类化合物（层状双羟基复合金属氧化物：LDH）–负载铜，银或金的纳米粒子作为环保催化剂催化氧化醇类物质具有较好的催化效果。特别是纳米金负载在水滑石化合物上在纯O2参与且无其他催化剂的条件下氧化醇类物质表现出较高的氧化性。通过降低纳米微粒颗粒的大小能够有助于改善纳米级催化剂的活性已经被广泛证实。但是，随着粒径尺寸的减少，用物理方法分离比如过滤或者离心，这一过程将变的非常困难。一种行之有效的解决办法就是研发出一种具有磁性的催化剂，一种很简单的分离方法只需用外部磁场的作用力就可以达到分离效果。从绿色化学的观点来看，发展高活性、高选择性、能再生利用的催化性已经成为可能。因此，这些年磁性分离纳米催化剂技术受到越来越广泛的关注，因为这项技术不仅可以减少一些辅料，能源以及时间的消耗，而且可以在一些重要的经济和环境领域收到成效。
有着核壳结构的磁性复合材料允许多种功能的个体结合成一个单一的纳米颗粒系统，并具有独特的应用方面的优势，特别是在生物医学和催化方面。然而，在某种程度上，将材料的直接组装在磁核表面是一个重大的挑战。在我们先前的工作中，在Fe3O4亚微米球体表面上首次涂覆一层很薄的碳层，之后又在其表面包覆了MgAl–LDH制成了一种抗癌剂，而Fe3O4错误!未找到引用源。@DFUR–LDH在这种抗癌剂中作为药物目标运输载体。李老师以及其团队人员用Fe3O4@MgAl–LDH通过逐层组装分层的LDH纳米片作为磁性矩阵负载在W7O24作为一种催化剂。这些具有核-壳结构的纳米复合材料同时具有磁性材料的磁化强度和LDH材料的多重功能。虽然如此，这些被报导的合成方法仍然具有多步及复杂的步骤。在这里，我们为该种新
型纳米级催化剂Fe3O4@MgAl–LDH@Au的生产制备设计出了一种简便的合成方法，包含有在Fe3O4纳米微球表面上定向生长的MgAl–LDH结晶体上负载纳米金离子，这种纳米微球兼备金纳米微粒的优良催化特性以及磁铁矿纳米微粒的超顺磁性。就我们所了解到的，这是第一个通过简单的共同沉淀方法，将MgAl–LDH片晶状的纳米级结晶体，直接竖直地定向于Fe3O4核心分子上的例子。通过分层的具有磁性的金属载体的纳米级催化剂的生产制备更进一步负载金属纳米粒子。
由图表1的图解知，Fe3O4@MgAl–LDH@Au的合成方案包含两个关键点。几乎所有的单分散的磁铁矿颗粒都通过无表面活性剂的疏溶剂的处理方法被前期合成。首先将Fe3O4悬浮纳米粒子的pH值调整到10，因此获得的完全带负电荷的Fe3O4纳米球粒子通过由界面成核作用带来的静电引力，很容易被附着上一层定向增长的碳酸盐–MgAl–LDH，随后晶体通过不断地滴加盐类和碱性溶液不断地生长。其次，金纳米粒子通过该种沉积-沉淀方法被有效地负载在如此组成的Fe3O4@MgAl–LDH的表面上（详见ESI）。
图1 Fe3O4(a)、Fe3O4@MgAl–LDH(b和d)及Fe3O4@MgAl–LDH@Au(c，e，f和g)的扫瞄式电子显微镜(a，b和c)、透射电镜(d和e)、高分辨透射电子显微镜(f)图像和X射线探测器光谱(g)。
图1描述了Fe3O4@MgAl–LDH@Au纳米催化剂在制备过程中不同阶段的扫瞄电镜/透射电镜的样本图片。Fe3O4纳米球（图1a）显示出一个表面光滑、平均直径在450纳米粒度的较小间隙尺寸的分布状态（图S1, ESI）。在直接附着有碳酸盐–MgAl–LDH以后(图1b)，蜂窝状的形态大小范围在100–200纳米的空间能够被清晰地观察到，LDH壳体由厚度大约为20纳米的交错的小片状体组成。有趣的是,MgAl–LDH壳体显现出显著的择优取向，与c轴并行，ab界面垂直于磁铁矿核心的表面，与先前的报导截然不同。也有类似的现象只是在报导的LDH影像及聚合物树脂小球阳离子交换中的多层氢氧化物的增长中被观察到。两个独立的纳米球微粒的透射电子显微镜摄影图片(图1d) 无容置疑的证实了Fe3O4核心被完全附着一层LDH纳米晶体的Fe3O4@MgAl–LDH@Au的核-壳结构。具体的讲，MgAl–LDH结晶体单分子层形成为大片的薄状的纳米片状颗粒，显现出厚度大约为20纳米，宽度大约为100纳米的边缘开裂的薄片状片晶，由磁铁矿核心增长至其外表面并垂直于Fe3O4的表面。获得的拥有表面积为43.3 m2 g，提供了足够的可接近的壳进入的边缘和结合点的LDH结晶体的核-壳结构的Fe3O4@MgAl–LDH纳米球微粒的表面蜂窝状的微结构能够使该种新型分层复合物负载金属的纳米球微粒。拥有该种结构形态，交错垂直地定向于MgAl–LDH纳米结晶体上，能够促进纳米级金属颗粒的定向负载，而避免可能的聚集。
Coprecipitation 共同沉淀，DP method 聚合方法 图表1 Fe3O4@MgAl–LDH@Au催化物的合成方法。
外文翻译--简易合成易回收的分层核壳Fe3O4
